The present invention relates broadly to devices for coating substrates and particularly to a device well suited for flame spraying various coating materials onto a substrate.
In the field of coating substrates, many different coating materials and devices for applying the material to a substrate have been developed. One such device is a flame spray gun which has a means for introducing a coating material, for example a powder, into a flame which then melts the coating material. The melted material is then carried by the flame to the substrate and adheres thereto. During the dwell time, i.e., the time during which the coating material is in the flame, the coating material is raised to an elevated temperature by the flame. At this elevated temperature, the coating material becomes molten so when it strikes the substrate it will adhere and cool thereon to form a layer of the coating material on the substrate.
In a combustion flame spray gun, the heating zone occurs within a combustion flame of a fuel such as acetylene, propane, natural gas or the like, with oxygen or air as the oxidizing agent. In a plasma flame spray gun, the heat is supplied by an electric arc flame and preferably by a free plasma flame, which issues from a nozzle after being heated by a high intensity electric arc.
In typical flame spray equipment, the coating material dwell time must be sufficiently long to achieve melting of the coating material so it will flow and adhere upon striking the substrate. Melting herein includes at least heat softening the surface of the particles of coating material. For most flame spray guns, because of the dwell time in the flame the coating material has a tendency to oxidize while in the flame. As a result of oxidation, the quality of the coating on the substrate achieved by such flame spray guns is not as high as may be desired.
It is known that higher velocity flames will propel the material at a faster rate, reducing the dwell time and, therefore, the degree of oxidation. Also, the higher velocity will cause the particles to flatten better at the substrate and to fill voids during buildup of the coating, resulting in coatings of higher density and quality. Plasma flame spray guns can provide higher velocity flames for producing coatings of high quality and low oxide content.
However, it is difficult to inject powdered flame spray materials uniformly into a high velocity flame and thus there are low heating efficiencies that cause low deposit efficiencies and erratic coating results. For example, some of the powder is accelerated along the cooler fringe of the high velocity flame and is heated insufficiently. Also, because of the short dwell time very high electric arc power is required for the plasma flame to heat the powder during the short dwell time, causing further problems with arc erosion of the internal components of the plasma gun, adding to maintenance time and expense.
Sometimes jets of high pressure air or inert gas are used to accelerate powder particles from a flame spray gun. However, these jets tend to cool the particles resulting in partially or completely solidifying the previously melted coating particles causing low deposit efficiency.
Accordingly, it is a primary object of the present invention to provide a flame spraying device which operates in a manner to minimize the oxidation of the coating material prior to its contacting the substrate.
It is a further object of the present invention to provide a flame spraying device which minimizes oxidation of the coating material while creating a denser and higher quality coating on the substrate.
Still further, it is an object of the present invention to provide a flame spraying gun with a higher deposit efficiency than can be achieved through conventional techniques.
The above objects of the invention are achieved by modifying a conventional flame spraying device by adding a means to accelerate the molten coating material after it leaves the flame spray gun so as to thereafter increase velocity and reduce the dwell time. The accelerator may take several forms each of which employs a combustion rocket with the gaseous products of combustion directed generally toward the path followed by the coating material as it travels from the flame spray gun to the substrate. In one form, the accelerator comprises two or more discrete rockets disposed around the spray nozzle and designed to aim the combustion product gases thereof generally toward though preferably at an acute angle or parallel to the path followed by the coating material. In another form, the accelerator rocket has an annular orifice disposed around the flame spraying gun nozzle and designed to direct combustion gases in a direction either parallel or at an acute angle to the direction of the molten coating material after it leaves the flame spraying nozzle. In some instances such as for coating the inside bore of a pipe the rocket may be approximately perpendicular to the flame of the flame spray gun.
The rocket accelerator(s) provide a high velocity hot gas stream which accelerates the molten coating material toward the substrate. Thus, by reason of acceleration, the dwell time is reduced. By reason of the lower dwell time, oxidation of the coating material is reduced. As the coating material is accelerated by a high velocity hot gas stream, the coating material does not cool excessively in transit between the gun nozzle and the substrate and the coating material strikes the substrate at a higher velocity than is achieved by a conventional flame spray gun. Both of these invention attributes contribute to a denser and higher quality coating being deposited on the substrate.